Often microscopes have built-in filter holders located above the light source that can be swung into place above the light or moved out of the way when not in use. The image below shows built-in filter holders that swing out of place.

Microscope with Swing-Out Filter Holders

Other microscopes will have a filter holder that slides directly into the condenser. The image below of the biological microscope shows where the filter can be placed in the slider for insertion in the condenser. Additionally, this filter slider can be removed and a darkfield slider or a phase contrast slider can be inserted in this condenser.

Microscope Condenser with Filter Slot

The final image below shows the same filter from above fully inserted into the microscope condenser.